Monostable multivibrators, sometimes known as one-shot multivibrators, are well known to those skilled in the art. One way of constructing a monostable multivibrator for use in digital systems includes utilizing an inverter and a NOR gate. One of the inputs of the NOR gate is connected to the output of the inverter, while the other input remains available to directly receive an incoming signal. The input of the inverter and the available input of the NOR gate are electrically connected to the same node.
A digital signal incident on the node will thus be applied to both an input of the inverter and an input of the NOR gate. As the signal is operated on by the inverter, the output thereof will be slightly delayed with respect to the original input signal. The output of the inverter is then received by the input of the NOR gate. Assuming that the node is ordinarily held at a logic low, a logic high input pulse of arbitrary length will cause the system described above to produce a logic high pulse of predetermined length at the output of the NOR gate. The duration of this pulse will be roughly equal to the time of the delay caused by the inverter. Any odd number of inverters may be used to increase the length of the delay, and/or capacitors may be connected to the outputs of some or all of the nodes to increase the delay.
While devices constructed in accordance with the foregoing are suitable in many conventional systems, shortcomings often become apparent, especially in high-speed or low-power applications.
During the delay created by the inverter, the voltage at the output of the inverter is gradually increasing in magnitude and in response to this voltage, the input MOSFET of the NOR gate begins to conduct current. This current path draws current from the output MOSFET of the NOR gate, reducing the efficiency of the output MOSFET as it charges its output node. Hence, the conventional monostable multivibrator has a gradually sloping output voltage waveform which is unable to efficiently optimize the voltage amplitude at the output of the multivibrator with respect to time. In other words, the system consumes unnecessary power between switching states due to resistive dissipation of energy because of current flow. Moreover, the resulting waveform at the output of the NOR gate lacks a sharp, distinct edge.
It is accordingly an object of the invention to provide a way to isolate the input MOSFET of a MOS logical device from voltage transients so that it can efficiently charge its output.
Another object of the invention is to provide means to quickly force a node to a given potential to reduce current flow and produce an output signal with well defined edges.
Yet another object of the invention is to provide a device which can provide a delayed signal synchronized to an input signal in a simplified construction.
These and other objects of the invention will become clear from the disclosure which follows.